It’s admirable to erase your system before you sell—especially with secure erasure—to avoid leaking personal data to someone who buys it or obtains the disk drive. While the odds are likely very low someone would be able to extract data (or be interested in it), you can try to reduce those odds to what is effectively zero.

Susan has a number of options of how to proceed, but there’s a bit of navigation along the way. She’s running 10.6.8 (Snow Leopard), which didn’t include Recovery HD. Security Options in her version of Disk Utility is grayed, because you can’t erase a disk from its startup volume. (That option is always unavailable for SSDs, but an older iMac won’t have an SSD.)

Here are the easiest ways to erase that drive securely by degree of difficulty:

Use Target Disk Mode. With two Macs that both have Thunderbolt or FireWire ports, connect them with the appropriate cable, and follow Apple’s instructions (either hold down the T key after restarting the one to erase or use Startup Disk on that computer before restarting it). The Mac to erase mounts as a drive, and Disk Utility can be used with it.

Install OSX on an external drive or borrow one with OS X installed. Booting off that drive will also allow erasing via Disk Utility.

Upgrade to a newer version of OS X and use Recovery HD. Snow Leopard with the Mac App Store installed should let you download at no cost a later version of OS X. All versions after Snow Leopard include and will install Recovery HD, from which you can then boot and run

iMac models released starting in 2010 can use Internet Recovery, but first have to be upgraded at least to Lion and some need a firmware update installed, so that’s not the simplest path to pursue.

A related question came in from Becky Steinke, who was trying to erase a 2008 MacBook. She tried to use the Recovery HD startup (holding down Command-R after restarting) and had no luck. Every 2008 and later MacBook Pro and MacBook can install Mac OS X 10.7 Lion, which should be able to install the Recovery partition. If possible, upgrade it to Lion, restart, and use Recovery HD to erase, or use one of the other options above.

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